This invention relates to an FM receiver comprising a radio frequency (RF) section, means for a down-conversion of a desired RF FM reception signal to an intermediate frequency (IF) FM signal, an IF section comprising a tunable IF filter, an FM demodulator for demodulating the baseband modulation signal of the IF FM signal, which demodulator is coupled to a baseband signal processing device, and an IF tuning control loop via which the FM demodulator is coupled to a tuning control input of the IF filter for an instantaneous tuning control of the IF filter by means of the baseband modulation signal.
An FM receiver of this type is known from the international PCT Application WO 88/08223, which corresponds to U.S. Pat. No. 5,155,862.
The meters for a down-conversion of a desired RF FM reception signal to an intermediate frequency (IF) FM signal of the known FM receiver comprise a tunable first mixer stage with which a desired RF FM reception signal from an RF FM reception range is converted down to a first intermediate frequency of 10.7 MHz. After a first IF selection in tiffs known FM receiver, the first intermediate frequency (IF) signal thus obtained is subsequently converted down again by means of a fixed second mixer stage, which results in a second IF signal having an average intermediate frequency of 700 kHz. After a second IF selection in said tunable IF filter, this second IF signal is applied to the FM demodulator for demodulating the second IF signal, which results in the desired baseband modulation signal. An output of the FM demodulator is coupled to the tuning control input of the tunable IF filter for a dynamic control of the tuning of the IF filter dependent upon the baseband modulation signal. Consequently, the resonance frequency of the IF filter instantaneously tracks the frequency modulation of the IF FM signal, so that the IF filter can have a comparatively narrow band, which enhances the selectivity.
In order to prevent that a neighbouring transmitter carrier, whose frequency approaches the carrier of the desired signal and which comes within the capturing range of the IF tuning control loop, gives rise to a tuning of the IF filter at this neighbouring transmitter, the tuning control of the known FM receiver is switched off at a given amplitude level of the tuning control signal, which is directly followed by a reset of the tuning of the IF filter at an initial value, for example, at the 700 kHz intermediate frequency. Switching off the tuning control of the IF filter and recapturing the IF tuning control at the desired carrier when it reappears in the passband of the IF filter tuned by means of said initial reset, gives rise to clearly audible noise peaks which may subsist through comparatively long periods and/or may lead to comparatively long muting periods if these noise signals are suppressed by means of a muting circuit.